Bites from ectoparasites, in particular fleas, can cause a hypersensitive response in animals. In particular, hypersensitive responses to fleabites is manifested in a disease called flea allergy dermatitis (FAD). Hypersensitivity refers to a state of altered reactivity in which an animal, having been previously exposed to a compound, exhibits an allergic response to the compound upon subsequent exposures. Hypersensitive responses include Type I, Type II, Type III and Type IV hypersensitivities. Type I hypersensitivity is described as IgE-mediated hypersensitivity in which an allergen induces cross-linkage of IgE bound to Fc receptors on the surface of mast cells. This cross-linkage results in the degranulation of the mast cells. Type II hypersensitivity is described as antibody-mediated cytotoxic hypersensitivity in which antibodies bind to cell surface allergens resulting in cell destruction via complement activation. Type III hypersensitivity is described as immune complex-mediated hypersensitivity in which allergen-antibody complexes deposit in various tissues and induce inflammatory responses. A delayed hypersensitive reaction includes Type IV hypersensitivity which is described as a cell-mediated hypersensitivity in which T lymphocytes (i.e., T cells) release cytokines that activate macrophages or cytotoxic T cells which mediate cellular destruction.
A Type I hypersensitive response usually occurs within about 2 to 30 minutes following exposure to an allergenic compound, which is usually a soluble allergen. Type II and III responses can occur from about 2 to 8 hours following exposure to an allergenic compound. Alternatively, in a delayed hypersensitivity response, the allergic response by an animal to an allergenic compound typically is manifested from about 24 to about 72 hours after exposure to the compound. During the 24-hour delay, mononuclear cells infiltrate the area where the agent is located. The infiltrate can include lymphocytes, monocytes, macrophages and basophils. Lymphokines (e.g., interferon-.gamma.) are produced which activate monocytes or macrophages to secrete enzymes (e.g., proteases) which cause tissue damage.
Foreign compounds that induce symptoms of immediate and/or delayed hypersensitivity are herein referred to as allergens. The term "allergen" primarily refers to foreign compounds capable of causing an allergic response. The term can be used interchangeably with the term "antigen," especially with respect to a foreign compound capable of inducing symptoms of immediate and/or delayed hypersensitivity. Factors that influence an animal's susceptibility to an allergen can include a genetic component and/or environmental exposure to an allergen. Animals can be de-sensitized to an allergen by repeated injections of the allergen to which an animal is hypersensitive.
FAD can have manifestations of both immediate and delayed-type hypersensitivity. Typically, an immediate hypersensitive response in an animal susceptible to FAD includes wheal formation at the site of a fleabite. Such wheals can develop into a papule with a crust, representative of delayed-type hypersensitivity. Hypersensitive reactions to fleabites can occur in genetically pre-disposed animals as well as in animals sensitized by previous exposure to fleabites.
Effective treatment of FAD has been difficult if not impossible to achieve. FAD afflicts about 15% of cats and dogs in flea endemic areas and the frequency is increasing each year. In a geographical area, effective flea control requires treatment of all animals. One treatment investigators have proposed includes desensitization of animals using flea allergens. However, reliable, defined preparations of flea allergens are needed for such treatments.
Until the discovery of the novel formulations of the present invention, flea allergens responsible for FAD had not been clearly defined. Whole flea antigen preparations have been used to diagnose and desensitize animals with FAD (Benjamini et al., 1960, pp. 214-222, Experimental Parasitology, Vol. 10; Keep et al., 1967, pp. 425-426, Australian Veterinary Journal, Vol. 43; Kristensen et al., 1978, pp. 414-423, Nord. Vet-Med, Vol. 30; Van Winkle, 1981, pp. 343-354, J. Amer. Animal Hosp. Assoc., Vol. 17; Haliwell et al., 1987, pp. 203-213, Veterinary Immunology and Immunopathology, Vol. 15; Greene et al., 1993, pp. 69-74, Parasite Immunology, Vol. 15); PCT Publication No. WO 93/18788 by Opdebeeck et al.; and Van Winkle, pp. 343-354, 1981, J. Am. Anim. Hosp. Assoc., vol. 32. Available commercial whole flea extracts, however, are unpredictable and, therefore, have limited usefulness.
Prior investigators have suggested that products contained in flea saliva might be involved in FAD and have also suggested methods to isolate such products: Benjamini et al., 1963, pp. 143-154, Experimental Parasitology, Vol. 13; Young et al., 1963, pp. 155-166, Experimental Parasitology 13, Vol. 13; Michaeli et al., 1965, pp. 162-170, J. Immunol., Vol. 95; and Michaeli et al., 1996, pp. 402-406, J. Immunol., Vol. 97. These investigators, however, have characterized the allergenic factors of flea saliva as being haptens having molecular weights of less than 6 kilodaltons (kD). That they are not proteins is also supported by the finding that they are not susceptible to degradation when exposed to strong acids (e.g., 6N hydrochloric acid) or heat. Some of the particular low molecular weight allergenic factors have also been characterized as being a highly fluorescent aromatic fraction (Young et al., ibid.). In addition, studies by such investigators have indicated that in order to be allergenic, such factors need to be associated with adjuvants and/or carriers, such as collagen or portions of the membrane used to collect the oral secretions. Moreover, the methods described to collect flea saliva factors were difficult and unpredictable. Furthermore the factors isolated by these methods were typically contaminated with material from the fleas, their culture medium or the skin-based membranes used to allow the fleas to feed.
Thus, there remains a need to more clearly define flea saliva allergens capable of inducing a hypersensitive response in animals. In addition, there remains a need to develop a method to collect substantially pure flea saliva allergens which provide predictable and less expensive preparations of allergens useful for desensitizing animals subject to, or having, FAD.